Danish Geotechnical Institute

Kongens Lyngby, Denmark

Danish Geotechnical Institute

Kongens Lyngby, Denmark

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Nermoen A.,IRIS - International Research Institute of Stavanger | Korsnes R.,University of Stavanger | Christensen H.F.,Danish Geotechnical Institute | Trads N.,Danish Geotechnical Institute | And 3 more authors.
47th US Rock Mechanics / Geomechanics Symposium 2013 | Year: 2013

Reservoir compaction is an effective mechanism for enhanced oil recovery (EOR) and a threat to well bore stability and installations resting on the sea floor. It is of great importance to understand the underlying physical and chemical processes in compacting formations in order to precisely manage the reservoir development. It is a common procedure to reduce the set of state variables by introducing an equivalent effective stress variable calculated from the overburden weight and a fraction of the pore pressure. The fraction of the pore pressure is termed the Biot effective stress coefficient. A precise determination of the effective stress is needed to predict compaction, since it is the effective stress that imparts deformation on the matrix. Within a reservoir, the overburden and pore pressure are measurable quantities, while the Biot stress coefficient and therefore the exact value of the effective stress itself remain unknown without the use of tailored experimental techniques. Here, we present the results from four independent testing methods for estimating the Biot stress coefficient of high porosity chalk. We discuss the microscopic interpretation of the Biot coefficient and show how the Biot coefficient may be pore pressure sensitive. As such, the attempt to reduce the number of variables by introducing an equivalent effective stress state with the usage of a constant coefficient is potentially flawed for wider ranges of the pore pressure. Copyright 2013 ARMA, American Rock Mechanics Association.


Koreta O.,Danish Geotechnical Institute | Myftaraga E.,POLIS University | Tanku E.,ALB STAR
Geotechnical Engineering for Infrastructure and Development - Proceedings of the XVI European Conference on Soil Mechanics and Geotechnical Engineering, ECSMGE 2015 | Year: 2015

In geotechnical engineering analysis, the uncertainties in the soil input parameters have always been a concern. Therefore, to take into account this issue, engineers are using reliability based design (RBD), which can deal with the probability of failure rather than only with the factor of safety. In this paper is presented a probabilistic analysis of a sheet pile wall penetrating clay, which has been analysed with the limit equilibrium method. It has been studied the impact of the variation of soil parameters (friction angle, cohesion) on the values of penetration depth, maximal bending moment, safety factor and probability of failure. In this structure, according to the failure mechanism, the soil is involved as load and also as resistance due to the active and passive earth pressures. © The authors and ICE Publishing: All rights reserved, 2015.


Alam M.M.,Technical University of Denmark | Hjuler M.L.,Danish Geotechnical Institute | Christensen H.F.,Danish Geotechnical Institute | Fabricius I.L.,Technical University of Denmark
Journal of Petroleum Science and Engineering | Year: 2014

Enhanced oil recovery by CO2 injection (CO2-EOR) is a tertiary oil recovery process which has a prospective for being used, at the same time, as an effective technique for carbon dioxide storage. There is a huge potential for additional oil production and CO2 storage in the North Sea depleted chalk reservoirs. North Sea chalk is characterized by high porosity but also high specific surface causing low permeability. A high porosity provides room for CO2 storage, while a high specific surface causes a high risk for chemical reaction and consequently for mechanical weakening. In order to address this issue we studied two types of chalk from South Arne field, North Sea: (1) Ekofisk Formation having >12% non-carbonate and (2) Tor Formation, which has less than 5% non-carbonate. We performed a series of laboratory experiments to reveal the changes in petrophysical and rock-mechanics properties due to the injection of CO2 at supercritical state. We analyzed these changes with respect to the differences in porosity, specific surface, pore stiffness, wettability, mineralogy and mechanical strength. We observed a 2-3% increase in porosity, a minor decrease of specific surface and consequently a small increase in permeability. A decrease in elastic stiffness is indicated by an increase of Biot's effective stress coefficient (α) by 1-2%. Nuclear Magnetic Resonance (NMR) data indicated no change in wettability and the samples remained water wet. We found that the effect of CO2 injection on both petrophysical and mechanical properties of chalk depends on carbonate content. Pure chalk with high carbonate content was found to be vulnerable to mechanical weakening due to CO2 injection, whereas, no significant mechanical effect was observed in the impure chalk of Ekofisk Formation. It should in this context be noted that the experiments spanned only 8 days, therefore long term effects cannot be ruled out. In spite of weakening of the chalk, we expect only minor mechanical effects, because the weakening also causes a lowering of effective stress due to an increase in effective stress coefficient. Extensive time-lapse monitoring strategies are required during a CO2-EOR process for the measurement of changes in reservoir properties that may cause deformation of and leakage from a reservoir. Results of this study will provide data for designing future monitoring strategies based on 4D seismic. © 2014 Elsevier B.V.


Alam M.M.,Technical University of Denmark | Hjuler M.L.,Danish Geotechnical Institute | Christensen H.F.,Danish Geotechnical Institute | Fabricius I.L.,Technical University of Denmark
74th European Association of Geoscientists and Engineers Conference and Exhibition 2012 Incorporating SPE EUROPEC 2012: Responsibly Securing Natural Resources | Year: 2012

Reservoir modeling and monitoring uses dynamic data for predicting and determining static changes. Dynamic data are achieved from the propagation velocity of elastic waves in rock while static data are obtained from the mechanical deformation. Reservoir simulation and monitoring are particularly important in enhanced oil recovery by CO2 injection (CO2-EOR) in chalk as, chalk reservoirs are vulnerable to compaction under changed stress and pore fluid. From South Arne field, North Sea, we used Ekofisk Formation chalk having approximately 20% non-carbonate and Tor Formation chalk having less than 5% non-carbonate. We studied difference in static and dynamic behavior. Furthermore, brine saturated data were compared with CO2 injected data to reveal the effect of supercritical CO2 injection in both static and dynamic elastic properties. We used strain gauges and LVDTs to measure static deformation. We observed lower dynamic elastic modulus for chalk with higher non-carbonate content at porosities lower than 30%. In 30% porosity chalk, dynamic compressional and bulk modulus were found significantly higher than the static modulus. Static measurements with LVDT were found lowest. The effect of CO2 injection was notable in dynamic elastic properties, while a possible change in static elastic properties was below detection limit.


Alam M.M.,Technical University of Denmark | Hjuler M.L.,Danish Geotechnical Institute | Hjuler M.L.,Geological Survey of Denmark | Christensen H.F.,Danish Geotechnical Institute | Fabricius I.L.,Technical University of Denmark
Journal of Petroleum Science and Engineering | Year: 2014

Enhanced oil recovery by CO2 injection (CO2-EOR) is a tertiary oil recovery process which has a prospective for being used, at the same time, as an effective technique for carbon dioxide storage. There is a huge potential for additional oil production and CO2 storage in the North Sea depleted chalk reservoirs. North Sea chalk is characterized by high porosity but also high specific surface causing low permeability. A high porosity provides room for CO2 storage, while a high specific surface causes a high risk for chemical reaction and consequently for mechanical weakening. In order to address this issue we studied two types of chalk from South Arne field, North Sea: (1) Ekofisk Formation having >12% non-carbonate and (2) Tor Formation, which has less than 5% non-carbonate. We performed a series of laboratory experiments to reveal the changes in petrophysical and rock-mechanics properties due to the injection of CO2 at supercritical state. We analyzed these changes with respect to the differences in porosity, specific surface, pore stiffness, wettability, mineralogy and mechanical strength. We observed a 2-3% increase in porosity, a minor decrease of specific surface and consequently a small increase in permeability. A decrease in elastic stiffness is indicated by an increase of Biot's effective stress coefficient (α) by 1-2%. Nuclear Magnetic Resonance (NMR) data indicated no change in wettability and the samples remained water wet. We found that the effect of CO2 injection on both petrophysical and mechanical properties of chalk depends on carbonate content. Pure chalk with high carbonate content was found to be vulnerable to mechanical weakening due to CO2 injection, whereas, no significant mechanical effect was observed in the impure chalk of Ekofisk Formation. It should in this context be noted that the experiments spanned only 8 days, therefore long term effects cannot be ruled out. In spite of weakening of the chalk, we expect only minor mechanical effects, because the weakening also causes a lowering of effective stress due to an increase in effective stress coefficient. Extensive time-lapse monitoring strategies are required during a CO2-EOR process for the measurement of changes in reservoir properties that may cause deformation of and leakage from a reservoir. Results of this study will provide data for designing future monitoring strategies based on 4D seismic. © 2014 Elsevier B.V.


Alam M.M.,Technical University of Denmark | Fabricius I.L.,Technical University of Denmark | Christensen H.F.,Danish Geotechnical Institute
Geophysics | Year: 2012

Deformation of a hydrocarbon reservoir can ideally be used to estimate the effective stress acting on it. The effective stress in the subsurface is the difference between the stress due to the weight of the sediment and a fraction (effective stress coefficient) of the pore pressure. The effective stress coefficient is thus relevant for studying reservoir deformation and for evaluating 4D seismic for the correct pore pressure prediction. The static effective stress coefficient n is estimated from mechanical tests and is highly relevant for effective stress prediction because it is directly related to mechanical strain in the elastic stress regime. The corresponding dynamic effective stress coefficient α is easy to estimate from density and velocity of acoustic (elastic) waves. We studied n and α of chalk from the reservoir zone of the Valhall field, North Sea, and found that n and α vary with differential stress (overburden stress-pore pressure). For Valhall reservoir chalk with 40% porosity, α ranges between 0.98 and 0.85 and decreases by 10% if the differential stress is increased by 25 MPa. In contrast, for chalk with 15% porosity from the same reservoir, α ranges between 0.85 and 0.70 and decreases by 5% due to a similar increase in differential stress. Our data indicate that α measured from sonic velocity data falls in the same range as for n, and that n is always below unity. Stress-dependent behavior of n is similar (decrease with increasing differential stress) to that of α during elastic deformation caused by pore pressure buildup, for example, during waterflooding. By contrast, during the increase in differential stress, as in the case of pore pressure depletion due to production, n increases with stress while α decreases. © 2012 Society of Exploration Geophysicists.


Alam M.M.,Technical University of Denmark | Hjuler M.L.,Danish Geotechnical Institute | Christensen H.F.,Danish Geotechnical Institute | Fabricius I.L.,Technical University of Denmark
Proceedings - SPE Annual Technical Conference and Exhibition | Year: 2011

Changes in chalk due to EOR by injecting supercritical CO 2 (CO 2-EOR) can ideally be predicted by applying geophysical methods designed from laboratory-determined petrophysical and rock mechanics properties. A series of petrophysical and rock mechanics tests were performed on Ekofisk Formation and Tor Formation chalk of the South Arne field to reveal the changes in petrophysical and rock mechanics properties of chalk due to the injection of CO 2 at supercritical state. An increase in porosity and decrease in specific surface was observed due to injection of supercritical CO 2. This indicates that a reaction between CO 2 enriched water and particles takes place which smoothens the particle surface. Accordingly, partial increase in permeability was also noticed. An effect is also observed from the decrease in pore-space stiffness, calculated from sonic velocity. No significant effect on wettability as indicated by NMR T 2 relaxation time was observed. Rock mechanics testing indicates that in 30% porosity chalk from the South Arne field, injection of supercritical CO 2 has no significant effect on shear strength and compaction properties, while there is probably a slight decrease in stiffness properties. For both the Tor Formation and Ekofisk Formation, flooding with CO 2 after waterflooding does not seem to affect the shear strength parameters. For the Tor Formation, the elastic deformation parameters seem to be reduced after CO 2 injection. In contrast, the effect on the elastic moduli in Ekofisk Formation is insignificant. The time dependent properties for both Tor Formation and Ekofisk Formation do not seem to be affected by CO 2 flooding in the relevant stress regime. Generally, the change in both petrophysical and rock-mechanics properties is insignificant in Ekofisk Formation, compared to the changes in Tor Formation, most probably due to the very little contact cement in Ekofisk Formation chalk. Time-lapse monitoring strategies may be required during a CO 2-EOR process for the measurement of changes in reservoir properties that may cause deformation of and leakage from a reservoir. Results of this study will provide data for designing future monitoring strategies based on 4D seismic. Copyright 2011, Society of Petroleum Engineers.


Alam M.M.,Technical University of Denmark | Fabricius I.L.,Technical University of Denmark | Hjuler M.L.,Danish Geotechnical Institute | Christensen H.F.,Danish Geotechnical Institute
46th US Rock Mechanics / Geomechanics Symposium 2012 | Year: 2012

The Effective stress coefficient is a measure of how chalk grains are connected with each other. The stiffness of chalk may decrease if the amount of contact cements between the grains decreases, which may lead to an increase of the effective stress coefficient. We performed CO2 injection in chalk, as this process could affect the grain contact cement. If this happens, the effective stress at the grain contacts in a reservoir will change according to the effective stress principle of Biot. In a p′-q space for failure analysis, we observed that a higher effective stress coefficient reduces the elastic region and vice versa. However, as the effective stress working on the rock decreases with increased effective stress coefficient, the reduction of elastic region will have less effect on pore collapse strength if we consider the change in the effective stress coefficient. This finding will help estimate a more precise failure strength of chalk during changed stress state and under the influence of chemically reactive fluids during production of hydrocarbon and geological storage CO2. Copyright 2012 ARMA, American Rock Mechanics Association.


Xu L.,Danish Geotechnical Institute | Kellezi L.,Danish Geotechnical Institute
Numerical Methods in Geotechnical Engineering - Proceedings of the 8th European Conference on Numerical Methods in Geotechnical Engineering, NUMGE 2014 | Year: 2014

Problems related to vibrations of foundations have required increased attention during the past two decades, and notable advances have been made during the past ten years. Recently contributions include new theoretical procedures for investigating the dynamic responses of foundations. A laboratory method has been presented describing the dynamic behavior of soil. As an alternative to the laboratory method, in order to compare the results of the investigation numerical method is applied to study the effects of the in-filled material Expanded Polystyrene (EPS) acted as a wave barrier on the behavior of the particle velocity transmitting in the subsoil under vibration. Two-dimensional axisymmetric models are used to simulate the test conditions. Absorbent boundaries are specified at the bottom and right side boundary. The physical damping due to viscous effect is taken into account by Rayleigh damping. © 2014 Taylor & Francis Group, London.

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